Electronically controlled pneumatic and hydraulic valves are used in a wide variety of applications. Typically, in these types of valves, there is some type of magnetized element that is responsive to a magnetic field producing element, such as an electrical coil (commonly called a voice coil). A valve member is coupled either to the magnetized element or to the magnetic field producing element. A portion (typically called a spool) of the valve member fits into a sleeve, which contains a number of openings. Lands on the spool are defined to completely cover, partially cover, or uncover associated ones of the openings in the sleeve, depending on the position of the valve member, the positioning of which is controlled at least in part by current through the magnetic field producing element.
Such positioning may also be influenced by other elements of valve design. For example, one or more springs are positioned and designed so that when there is no current through the magnetic field element, the lands cover associated openings. The valve is therefore in a closed position when “off” in this example. As the current to the magnetic field producing element is modified, the valve member will move and therefore the openings will be partially or completely uncovered by the associated lands. In some valves, the valve member can move in both “positive” and “negative” directions. Examining a single set of openings and an associated land for instance, in a positive direction, coupling of fluid occurs from this single set of openings to a second set of openings, and in a negative direction, coupling of fluid occurs from the single set of openings to a third set of openings. A positive current will cause movement in the positive direction, and a negative current will cause movement in the negative direction. It is noted that the above description is merely exemplary and should not be construed to be limiting in any way.
In order to reduce unwanted leakage yet maintain relative ease of manufacturing, the lands are typically somewhat larger than associated openings. What this means is that there is some amount of positive or negative current that has to be applied in order to cause the valve member and lands to move enough to begin uncovering associated openings. Consequently, there is a range of positions in which modification of the input current to the magnetic field element causes no associated modification in valve output. Valve output may be quantified through, e.g., an increase in the percentage of aperture area created by uncovering the openings. This phenomenon of modification of input current but no modification of valve output is called a dead band, or more specifically a mechanical dead band. Dead bands mean that control of the valve is not as accurate as it could be. Therefore, it would be beneficial to reduce or eliminate this dead band.